What is nullification?
Zeroing is the deliberate electrical connection of open conductive parts of electrical installations that are not in a normal state under voltage, with a neutral neutral point of the generator or transformer, in three-phase current networks; with a ground-faulted terminal of a single-phase current source; with the grounded point of the source in direct current networks, performed for electrical safety purposes. Protective nullification is the main measure of protection against electric shock. current with a possible touch in electrical installations up to 1 kV with deadly grounded neutral.
Today, our life is difficult to imagine without the daily operation of all kinds of electrical appliances. However, the practical use of current is unsafe without protective systems. There are cases when security devices (plugs, automata, etc.) may not work, resulting in damage to the internal insulation and there is an increased voltage on the metal casing of the equipment. To protect a person from possible electric shock during the operation of electrical appliances and household appliances, various protective measures have been developed, including zeroing. This article is written with the purpose of explaining to the reader what is the peculiarity of zeroing, as a way of protecting electric grids, in what cases is applied and how it differs from protective grounding.
Zeroing is used to ensure the electrical safety of systems with PEN, PE or N conductors. These include networks with a blindly grounded neutral: TN-C, TN-S and TN-CS. The main difference in the organization of zeroing for these systems is in the scheme of connecting zero protective and working conductors.
TN-C nulling system
The system of zeroing TN-C for today refers to the obsolete, since it prevails in the buildings of the old housing stock. It is characterized by the presence along the entire length of the zero protective and zero working conductor PEN. Used for power supply in three-phase networks. It is forbidden for group and distribution single-phase networks. This system is quite simple in the organization, but does not provide sufficient level of electrical safety, which makes it impossible to use it in the construction of new buildings.
TN-CS nulling system
It is an improved version of the TN-C zeroing system for electrical safety in single-phase networks. At the branch point of the three-phase line, the single-phase combined PEN conductor is divided into PE and N conductors, bringing them to single-phase consumers. This system of zeroing, with a relatively small rise in price, is distinguished by a higher level of security.
TN-S nulling system
It is considered the most perfect and safe scheme of zeroing. The principle of operation is based on the separation along the entire length of the zero protective and zero working conductors. To the zero protective conductor PE all metal elements of the electrical installation are connected. In order to avoid re-earthing, a transformer substation having a basic grounding is arranged.
Electrical safety at zeroing
Using the scheme of protective zeroing, it is important to take into account that the current in the short circuit must reach a value sufficient to trigger the electromagnetic release of the circuit breaker or the fuse insert fuse. Otherwise, the fault current will freely flow through the electrical circuit, which will lead to an increase in the voltage in the damaged area and on all the zeroed elements of the electrical installation to a value at which the probability of electric shock from the body of the device will increase manifold. It turns out that the reliability of the system of zeroing is determined for the most part by the reliability of the zero protective conductor used, to which the increased requirements are respectively presented, see points 1.7.121 - 1.7.126 of the code. Carefully laid zero wire should differ in color in the form of yellow bands on a green background. In addition, it is necessary to constantly monitor the health of his condition. It is forbidden to mount protective equipment of the electrical installations to the neutral wire, which, if triggered, can lead to its damage. Connections of zero wires to each other and to the metal elements of the electrical installation, accessible for users to touch, should guarantee a reliable contact and be able to be inspected, see point 1.7.39, 1.7.40 of the IPPE-7. The resistance value in the bolted connection with the parts of the electrical installation should not exceed 0.1 ohms. Control over the resistance of the "phase-zero" loop is carried out at the stage of acceptance works, during major overhaul and reconstruction of the network, as well as in the time limits established in the regulatory and technical documentation.The measurements in the disconnected electrical installation are carried out with a voltmeter-ammeter, the value of the neutral resistance of the neutral and the repeated earthing switches, the time dependence of the automatic protection devices against the short-circuit current are subject to constant monitoring.
To reduce the electric shock, in the event of a ground wire break, it is recommended to perform repeated earthing with a resistance of no more than 30 ohms every 200 m of the line and supports, for which natural earthing switches are predominantly used.
Normalization of zeroing
The technical requirements for the organization of safety nets are defined by the following documents:
- Rules for the installation of electrical installations (PUE), chapter 1.7,
- GOST R 50571.5.54-2013 (para. 543),
- GOST 12.1.030-81 (item 7).
The mechanism of zeroing is based on automatic disconnection of the damaged network section, the time of which should not exceed the values according to the point 1.7.79 of the IPPE-7.
The maximum allowable time of a protective automatic shutdown for the TN system
|The nominal phase voltage U o , V||Shutdown time, s|
|more than 380||0.1|
Zero working and protective conductors must have a resistance sufficient to trigger protection. The active and inductive impedances of the conductors form the impedance of the "phase-zero" loop. The active resistance of the conductors depends on their length, the resistivity of the material and the cross-section. Inductive resistances are distinguished for conductors made of copper and steel. In a steel wire, they are inversely related to the current density and the ratio of the perimeter to the cross-sectional area of the conductor. Inductive resistance of steel conductors is higher than copper conductors. In clause 1.7.126 of the EMP-7, the smallest cross-sectional area of the protective conductors is installed for cases when they are made of the same material as the phase conductors. The cross-sections of protective conductors from other materials should be equivalent in conductivity to the above.
The smallest cross-sections of protective conductors
|Cross-section of phase conductors, mm2||The smallest cross-section of protective conductors, mm2|
|S ≤ 16||S|
|16 <S ≤ 35||16|
|S> 35||S / 2|
A two-wire line consisting of a working and protective conductor forms one large coil, the mutual inductance of which (the recommended value for calculations is 0.6 Ohm / km) depends on the length of the line, the diameter of the wires and the distance between them. Resistance of grounding of the neutral of the power source should not exceed 2, 4 and 8 ohms, respectively, with line voltages of 660, 380 and 220 V of the three-phase current source, see point 1.7.101 of the IPPE-7. An increase in the short-circuit current is achieved by lowering the resistance of the transformer and the loop, for which the triangle-star circuit is used. The windings of powerful transformers and so have little resistance. The smaller resistance of the zeroing lines is achieved by performing them short and simple, increasing the cross-section of the conductors, replacing the steel conductors with those made of non-ferrous metals with a low inductive resistance. The maximum resistance of the neutral protective conductor must not exceed twice the resistance of the phase conductor. By reducing the distance between them, reduce the external inductive resistance. Reducing the resistance of the repeated earthing switches and approaching them to the load nodes, helps to reduce the current on the zeroed parts of the equipment. The connection to the zero conductor of all grounded metal structures of the building increases the potential of the floor surface on which the person stands, and thus significantly reduces the touch voltage to a value approximately from 0.1 to 0.01 U.sup.
Application of zeroing
Zoning is performed at industrial facilities, often with a power source located in the building (generator or transformer), to ensure the safety of operation of electrical installations for various purposes and to increase noise immunity in their operation. According to the requirements of paragraph 1.7.101 of PES-7, the zeroing of electrical installations should be performed: - at a voltage of 380 V and above AC and 440 V and above DC in all electrical installations; - at rated voltages above 42 V, but below 380 V AC and above 110 V, but below 440 V DC - only in rooms with increased danger, especially dangerous and in outdoor installations. All electrical equipment of industrial facilities is connected to a common ground loop and connected to each other by a metal ground bus. A complete list of parts to be nullified is presented in Chapter 1.7 of the Electrical Installation Rules (PUE-7). There is also a list of electrical equipment, the deliberate zeroing of which is not required. For electrical protection of housing facilities, zeroing is practically not used. In new buildings, grounding is organized centrally. Modern electrical appliances have a plug with three contacts. One of the contacts is connected to the housing. Grounding for a single apartment is to connect the earths of the housings and parts of household appliances. The need for zeroing in this case disappears. The houses of the old housing stock, usually connected via the TNC system, may not have a grounding at all. Modernization of electric networks of similar houses should be handled by a specialized electrical company. However, often tenants of such houses resort to the arrangement of the forbidden in this case zanleniya, which is not a safe way of electrical protection for the residential sector. The requirements for the organization of a protective zero-pointing system, as already mentioned, are defined in regulatory documents. However, in the process of realizing this method of protecting electric networks, mistakes are sometimes made that prevent its direct purpose. It is a mistake to think that it is better to ground on a separate circuit from the neutral conductor, because there is no resistance of the long PEN conductor from the electrical appliance to the substation earthing switch. However, in practice, the ground resistance is much greater than that of a long wire. If the phase falls on the installation housing grounded in this way, the fault current may not be sufficient to trigger automatic protection of the mains. In this case, the voltage on the housing reaches a dangerous value for the user. Even with the use of a small-size circuit breaker, it is not possible to provide the required time for automatic disconnection of the faulty line from the network.
The difference between zeroing and earthing
In their purpose, grounding and zeroing are in many ways alike - they provide protection for the user of the electrical installation from electric shock. However, the methods and principle of organizing such protection are different. Ensuring the electrical safety of networks using the system of zeroing is discussed in detail in the previous sections of the article. The protective earthing is based on the forced connection of electrical installations to the ground in order to reduce the touch voltage to a safe value. Excess current flowing to the body of the electrical installation is taken directly to the ground (on the grounding part). A grounding loop of a triangular configuration whose resistance should be smaller than the rest of the circuit is installed as an earthing switch. The difference between zeroing and earthing is as follows:
- in the way to ensure the protection of electrical networks: grounding - reduces touch voltage, zeroing - disconnects the damaged electrical installation from the network, which virtually eliminates electric shock and, from this point of view, is a more effective means of protection for use in industrial plants. However, if we talk about the reliability of protection in the process of operation, the nullification is inferior to grounding because of the greater probability of damage to the integrity of the zero wire and the possible variation of the resistance of the "phase-zero" loop.
- systems of application: grounding is used exclusively for protection of networks with isolated neutral (TT and IT systems), zeroing - in networks with a deafely grounded neutral TN-C, TN-S and TN-CS, where PEN, PE or N conductors are present.
- By type of arrangement: in terms of simplicity and accessibility of the arrangement, nullification is a more complex and time-consuming method of protection, requiring technical knowledge and skills for the correct definition of the method and the middle point of the zeroing. In the case of protective earthing, separate parts of the susceptor are connected to ground, for which it is sufficient to apply instructions to electrical appliances.
The role of zeroing when working with electrical installations in industrial plants can not be overestimated. By disconnecting the damaged installation from the network in the event of an insulation breakdown, zeroing out acts as a reliable way of protecting a person from possible electric shock. To effectively ensure electrical safety, it is necessary to strictly comply with the design of the elements of the system of nullification to the standards reviewed, as well as careful and constant monitoring of their condition. The use of zeroing or grounding depends on the necessary method of protecting various electrical network systems.